Add default authentication failure message

[lunkwill42]

The current API implementation unintentionally responds with an empty "500" message when a user authentication fails (i.e. the user exists, but the challenge response is incorrect). The correct response, according to Zino 1 codebase is "Authentication failure", not an empty string.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions[bot]]

Test results

    3 files      3 suites   31s :stopwatch: 296 tests 296 :heavy_check_mark: 0 :zzz: 0 :x: 888 runs  886 :heavy_check_mark: 2 :zzz: 0 :x:

Results for commit f5bf514d.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.77%. Comparing base (8c40d0d) to head (f5bf514).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #210 +/- ## ======================================= Coverage 98.77% 98.77% ======================================= Files 41 41 Lines 5457 5458 +1 ======================================= + Hits 5390 5391 +1 Misses 67 67 ```

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